The present invention relates generally to treatment of sewage. More particularly, the present invention relates to a method of reclaiming water and usable and safely disposable brine concentrate from domestic sewage.
A household water supply usually contains several hundred milligrams per liter (mg/L) of minerals in the form of cations (e.g., sodium, magnesium and calcium, etc.) and anions (e.g., chloride, sulfate and bicarbonate, etc.). Subjected to the multiple uses in a home, the used water becomes a suspension of bathroom and kitchen wastes along with laundry wastes and cleaning compounds of multiple origins. Solvents, pigments, and pesticides are often disposed xe2x80x9cdown the drain.xe2x80x9d In addition to these substances, the used water also contain microgram amounts of several minerals (e.g., copper, cadmium, zinc, nickel, lead, chromium and iron) dissolved from household utensils, from copper and galvanized pipes and from lead and solder in the plumbing of older homes and apartment buildings. Domestic sewage will also contain minute, but increasing amounts, of antibiotics, hormones, and other health-related substances stemming from the growing medical and pharmaceutical industries.
In the prior art, domestic sewage can be reclaimed by using an activated sludge secondary treatment process followed by a microfiltration process and a reverse osmosis (RO) process. Water reclaimed by these prior art methods may re-enter the household water supply to be reused. Among these prior art processes, the reverse osmosis process is particularly important because it removes salt increment (i.e., the increase in total dissolved solids concentration associated with each use) from the treated water. Removal of salt increment is key for the sustainability of any multiple-pass water reuse scheme. If these added salts are not removed prior to reuse, the dissolved solids concentration of the reclaimed water will steadily increase, reducing reuse options. The reverse osmosis process is also important because it removes heavy metal ions from the reclaimed water and prevents heavy metal ions from re-entering the household water supply.
One problem of the prior art activated sludge/microfiltration/RO process is that it is costly. It has been estimated that a prior art activated sludge/microfiltration/reverse osmosis process uses more than 900 kWh of energy and costs more than U.S. $1200 to reclaim one million liters of sewage. Another problem of the prior art process is that heavy metals in the sewage will become highly concentrated along with other ions in the brine. The resultant high concentration of heavy metals adds to the hazards and costs of brine disposal.
In view of the foregoing, there exists a need for a wastewater treatment and reclamation process that is energy and cost effective.
The present invention provides a process of treating and reclaiming wastewater that is cost-effective. In addition, the end products of the reclamation process do not contain any toxic chemicals. Indeed, the end products of the reclamation process may be used in an economically and commercially beneficial way to help lower the cost of wastewater reclamation.
According to one embodiment of the invention, the treatment and reclamation process includes an intensely anaerobic methane producing process followed by a highly aerobic algal-based waste oxidation process, a Dissolved Air Flotation (DAF) process, a Slow Sand Filtration (SSF) and/or other filtration process (e.g., microfiltration, nanofiltration, ultrafiltration, etc.), a disinfection process and finally a Reverse Osmosis (RO) process. The anaerobic process precipitates most heavy metals as metal sulfides, and the aerobic algal-based wastewater treatment process entails the use of algae to adsorb the remaining heavy metals. The DAF process removes suspended solids and algae from the treated sewage. The SSF process ensures that substantially all suspended solids and microorganisms are removed from the treated sewage. The disinfection process eliminates microorganisms that are not already filtered out. Finally, the reverse osmosis process removes the remaining dissolved solids from the treated sewage to produce permeate water and a high salinity concentrate (e.g., brine). The permeate water is potable and the high salinity concentrate is substantially free of heavy metal ions. Preferably, the high salinity concentrate is used to cultivate halophilic algae, such as Dunaliella, which have a high commercial value. Thus the present invention provides for a method of reclaiming both water and high salinity concentrate from sewage. The present invention can also be used to reclaim other types of wastewater.
In furtherance of the present embodiment, the wastewater treatment and reclamation process removes calcium hardness from the sewage. Removal of calcium hardness xe2x80x9csoftensxe2x80x9d water and minimizes the pretreatment required prior to reverse osmosis thus extending the useful life of the membranes used in the reverse osmosis process.